This invention relates to a rechargeable battery charging circuit, and more particularly to a charging circuit capable of supplementary charging the rechargeable battery as close as possible to full charge.
It is common knowledge that a rechargeable battery is a battery which may be used repeatedly by recharging. When recharging, it is desirable to charge the rechargeable battery, in so far as possible, to its full capacity. A battery which is not fully charged is charge deficient, cannot output its intrinsic capacity, and cannot drive a load for long periods.
On the other hand, if the battery is over-charged, problems such as battery performance degradation and lifetime shortening are invited. Nevertheless, it is difficult to charge a rechargeable battery to its fully charged state without under-charging or over-charging.
In particular, when rapid charging is performed using large charging currents to reduce charging time, it is easy to over-charge the battery. For this reason, a full charge sensor, which detects when the battery reaches full charge and terminates charging, is well known. It has been observed that battery voltage and temperature rise when the rechargeable battery reaches full charge. Hence, the full charge sensor detects battery voltage or temperature rise to determine full charge. However, it is also well known that the effects of the ambient temperature during charging introduce variation in the degree of battery voltage and temperature rise. Consequently, in a system where full charge is determined by detecting when the battery temperature reaches a set value, there is a tendency, when the ambient temperature is low, for the battery to reach full charge without the battery temperature reaching the set value resulting in over-charging. For this reason, it is not uncommon to establish a set temperature value low enough to insure that over-charging will not occur even at low ambient temperatures.
Since the full charge sensor is set in this manner to prevent over-charging under various conditions, the battery cannot fully charge under all conditions. In particular, among rechargeable batteries, the nickel metal hydride battery is weaker with respect to over-charging and more subject to degradation than nickel cadmium and lead storage batteries. Consequently, this property must be considered and charging must be terminated early in the case of rapid charging.
In order to fully charge a rechargeable battery as rapidly as possible without over-charging, a method is known wherein supplemental charging with a very small current is performed after rapid charging close to full charge. For example, In Japanese Non-examined Patent Publication No. 4-121031 issued Apr. 22, 1992 discloses a charging circuit which performs supplementary charging for a fixed period after rapid charging of the rechargeable battery. However, this charging circuit still has the drawback that under-charging and over-charging can occur due to the effects of ambient temperature. This is because the supplementary charging is performed for a fixed period independent of the ambient temperature. As described before, variation in ambient temperature gives rise to variation in the degree of battery voltage and temperature rise at full charge. Therefore, there is also variation in the charge capacity of the battery after termination of rapid charging. Namely, the charge capacity after rapid charging is not a fixed value. For example, if one battery is charged at room temperature and a second battery is charged at a temperature below room temperature, even if the charging capacity is the same, the lower the ambient temperature the lower the output discharge capacity of the second battery. In other words, it is known that the second battery is not fully charged. Consequently, because of charge capacity variation during rapid charging due to ambient temperature effects, supplementary charging for a fixed period as described in the above disclosure has the drawback that the final battery charge varies depending on the ambient temperature.
It is thus an object of the present invention to provide a charging circuit which solves the above mentioned problems by adjusting the supplementary charging capacity according to the ambient temperature, and thereby charge a battery to full charge independent of the ambient temperature.